The next generation mobile wireless communication system, which is referred to as “5G,” will support a diverse set of use cases and a diverse set of deployment scenarios. 5G will encompass an evolution of today's 4G networks and the addition of a new, globally standardized radio-access technology known as “New Radio” (NR).
The diverse set of deployment scenarios includes deployment at both low frequencies (100s of MHz), similar to LTE today, and very high frequencies (mm waves in the tens of GHz). At high frequencies, propagation characteristics make achieving good coverage challenging. Narrow-beam transmission and reception schemes will be needed at higher frequencies to compensate for the corresponding high propagation loss. For a given communication link, a beam can be applied at both the transmission and reception point (TRP) and the user equipment (UE). The combination of a beam at the TRP and a beam at the UE is referred to here as a beam pair link (BPL). The task of the beam management procedure is to discover and maintain beam pair links.
In the example illustrated in FIG. 1, a BPL 116 has been discovered and is being maintained by the network. A BPL between TRP 122 and UE 120 (here including both the TRP beam 106 and UE beam 112) is expected to be discovered and monitored by the network using measurements on downlink reference signals used for beam management, e.g., CSI-RS (CSI-RS has been agreed in 3GPP as beam reference signal for New Radio (NR)). The CSI-RS for beam management can be transmitted periodically, semi-persistently, or aperiodically (e.g., event triggered) and they can be either shared between multiple UEs or be UE-specific. In order to find a suitable TRP beam (e.g., among TRP beams 102, 104, 106, 108, 110) the TRP 122 transmits CSI-RS in different TRP transmit (TX) beams on which the UE performs Reference Signal Received Power (RSRP) measurements and reports back the N best TRP TX beams (where N can be configured by the network). Furthermore, the CSI-RS transmission on a given TRP beam can be repeated to allow the UE 120 to evaluate suitable UE receive (RX) beams (e.g., among UE beams 112, 114). For each BPL, the UE 120 remembers the best UE RX beam, and whenever the TRP 122 transmits signals in a given BPL, the UE 120 applies the corresponding UE RX beam.
There are basically three different implementations of beamforming, applicable both at the TRP and at the UE: analog beamforming, digital beamforming, and hybrid beamforming. Each implementation has its pros and cons. Digital beamforming is the most flexible solution but also the costliest due to the large number of required radios and baseband chains. Analog beamforming is the least flexible but cheaper to manufacture due to reduced number of radio and baseband chains. Hybrid beamforming is a compromise between the analog and digital beamforming. One type of beamforming antenna architecture that has been agreed to study in 3GPP for the NR access technology is the concept of antenna panels, both at the TRP and at the UE. A panel is an antenna array of dual-polarized elements with typically one transmit/receive unit (TXRU) per polarization. An analog distribution network with phase shifters is used to steer the beam of each panel. FIG. 2 illustrates two examples of panels 202, 206 (a two-dimensional panel 202 to the left and one-dimensional panel 206 to the right, having dual-polarized elements 204), where each panel is connected to one TXRU per polarization.
It is expected that the phase and amplitude is known for each element within an analog array for both TX and RX, i.e. an analog array can be assumed to be calibrated. Hence, reciprocity can be used to find a good RX beam in case a good TX beam has been found, and vice versa.
The CSI-RSs for beam management are expected to cover the whole bandwidth in order to get a wideband sounding of the channel.